The present invention relates to a method of cleaning out an underground well and to an apparatus for applying said method.
When well bottom deposits, particularly sandy sediments of unconsolidated formations, are present, a first method consists in using drilling apparatus to recondition the production tubing at stratum level. The application of this method is very onerous because the equipment installed has to be dismantled and a new completion is necessary after the cleaning operation. In order to avoid having to resort to such heavy "workover" operations, various means are available for the concentric maintenance of well bottoms.
In particular, from U.S. Pat. No. 4,671,359 a cleanout system is known for extracting sediments from a lost casing (with or without integrated gravel packing), this system requiring a rigid tubular extension lowered by cable and anchored in a suitable receptacle to the bottom of the production tubing. A continuous flexible tube known as "coiled tubing" is lowered through this extension, carrying at its end nozzles distributing cleaning fluid which is supplied from the surface and the flow of which is directed towards the wall of the well bottom. The sediment is entrained in the stream of fluid pumped to the surface through the flexible tube, or more precisely the sediment rises to the surface through the annular space between the coiled tubing and the production tubing.
In a system of this kind the addition of this tubular extension to the production tubing and the use of the coiled tubing technique comprising a continuous flexible tube bring about an increase in the speed of the fluid and thus improve the evacuation of sediment via the annular space between the coiled tubing and said extension.
Nevertheless, this system has disadvantages.
A first disadvantage consists in that the circulation of the cleaning fluid exerts a back-pressure on the stratum, which is often friable and very sensitive (loss, emulsion, precipitate, etc.)
A second disadvantage consists in that the method requires a preliminary cable operation, the limitations of which in respect of length, load and deviation are well known.
A third disadvantage relates to formations under subhydrostatic conditions or depleted formations; through the back-pressure which it develops the system makes it necessary to use prepared fluids of low relative density which are compatible with the formation; this may then become prohibitive.
The extensive development of deviated and highly deviated wells and horizontal drains poses numerous new problems in connection with cleaning through the simple fact that the direction and speed of flow of the fluid evacuating solid matter by known, conventional methods and apparatus cannot oppose the forces tending to disintegrate the stratum.
From U.S. Pat. No. 4,744,420 an apparatus is thus also known for removing solid accumulations, such as sandy or other aggregates, from highly deviated or horizontal underground wells. This apparatus comprises a train of concentric tubes intended to be inserted into the well, one of said tubes being used to supply the working fluid to the well bottom and the other for returning the fluid loaded with sediment. At the end of the train the apparatus is also provided with a hydroejector which projects a part of the working fluid onto the sediment before applying suction to it.
Apart from the disadvantages already mentioned, the tube train is injected into the well after the existing completion has been dismantled. In addition, the apparatus cannot work continuously on a large area of accumulated sediment without extensive intervention, such as the addition of extra concentric tubes.
The use of this method disturbs the completion, and this is a major disadvantage.